In recent years, in order to enhance convenience, small-sized electronic appliances have been miniaturized, reduced in weight and made portable. Following this, batteries to be used for these appliances are being demanded to realize miniaturization, weight reduction and slimness more and more from now on.
In comparing energy capacities per unit mass, an energy capacity of a single substance of lithium is large and excellent as compared with energy capacities of other metals. For that reason, there have hitherto been reported a number of studies regarding lithium secondary batteries. However, the lithium secondary batteries involve a problem in safety. Also, lithium is restricted in natural resources and is expensive.
On the other hand, magnesium is abundant in natural resources and much more inexpensive as compared with lithium. Also, metallic magnesium is relatively large in ionization tendency and large in quantity of electricity per unit volume capable of being extracted by an oxidation reduction reaction. Moreover, when used for batteries, high safety can be expected. Accordingly, the magnesium battery is a battery capable of compensating drawbacks of the lithium secondary battery. As seen in such an example, metallic magnesium and a magnesium ion are a very promising material as an electrode active material in electrochemical devices and a charge carrier in electrolytic solutions, respectively.
In designing an electrochemical device using metallic magnesium or a magnesium ion, selection of an electrolytic solution is extremely important. For example, as a solvent constituting the electrolytic solution, not only water or protonic solvents but aprotonic organic solvents such as esters, acrylonitrile or the like cannot be used. This is because when such a solvent is used, a passivation film which does not allow a magnesium ion to pass therethrough is formed on the surface of metallic magnesium. A problem of the formation of this passivation film is one of bars in putting a magnesium secondary battery into practical use.
As an electrolytic solution which is free from a problem of the formation of a passivation film and which is capable of electrochemically utilizing magnesium, ether solutions of a Grignard reagent (RMgX, wherein R is an alkyl group or an aryl group; and X is chlorine, bromine or iodine) have been known from of old. When such an electrolytic solution is used, metallic magnesium can be reversibly deposited and dissolved. However, there is involved a problem that an oxidation decomposition potential of the electrolytic solution is low as about +1.5 V relative to an equilibrium potential of metallic magnesium so that its potential window is insufficient for use in electrochemical devices (see the description of D. Aurbach, et al., Nature, 407, pages 724 to 727 (2000) (pages 724 to 726 and FIG. 3) as described later).
On the contrary, in 1990, Hoffman, et al. in The Dow Chemical Company found an ether solution of Mg(ZR4)2 (wherein Z is boron or aluminum; and R is a hydrocarbon group) as an electrolytic solution capable of electrochemically utilizing magnesium (see U.S. Pat. No. 4,894,302 (columns 1 to 10 and FIG. 1) and T. D. Gregory, et al., J. Electrochem. Soc., 137, pages 775 to 780 (1990) (pages 775 to 780, Table 3 and FIG. 6) as described later). Also, in 2000, Aurbach, et al., in Bar-Iran University found a tetrahydrofuran (THF) solution of Mg(ZRnX4-n)2 (wherein Z is born or aluminum; R is a hydrocarbon group; X is a halogen; and n is from 0 to 3) (see JP-T-2003-512704 (pages 12 to 19 and FIG. 3) and D. Aurbach, et al., Nature, 407, pages 724 to 727 (2000) (pages 724 to 726 and FIG. 3) as described later). Not only they performed deposition and dissolution of metallic magnesium, but they made a prototype of a magnesium secondary battery and successfully measured charge and discharge thereof.
On the other hand, JP-A-2004-259650 (pages 4 and 5 and FIG. 1) as described later reports that the problem of a low oxidation potential of an electrolytic solution can be solved by using an ether solution of an aromatic Grignard reagent RMgX wherein R is an aryl group (wherein X is chlorine or bromine).